Question

Alleles are
A. Different molecular forms of a gene
B. Heterozygotes
C. Different phenotype
D. True breeding Homozygotes

Answer 1

Hint: Alleles are variations or forms of a same gene located at a specific chromosomal site. Alleles are generally present in pairs and control the same character or trait. Therefore, we can say alleles are an alternative form of a gene.

Complete answer: A gene is a specific nucleotide sequence that encodes for a particular protein. Alleles are variations of the same gene. Alleles arise by mutation of either a single base pair or several hundred base pairs. They are present in pairs – one allele inherited from the father and the other from the mother. Alleles influence the same phenotype or character. For example, consider a phenotype – eye colour. Colour of the eye can be black, brown, blue or even green. Therefore, blue, black, brown and green are the alleles of the gene that influence eye colour. Thus, we can say that alleles are the different molecular forms of a gene.
Heterozygotes are organisms that contain two different alleles of a gene. The organism will be heterozygous for that particular gene. Heterozygosity is seen in diploid organisms, where one allele is received from the father and the other from the mother. Out of the two alleles present, one allele generally dominates the other one and is expressed in the off-spring. It is known as the dominant allele and the other allele, the recessive allele. Dominant traits are denoted by capital letters, while recessive ones by small letters. Thus, a heterozygote individual can be denoted as Aa.
Phenotypes are visible traits. They are the physical characteristics of an individual like, hair colour, eye colour, skin colour, etc. Phenotypes are controlled by alleles; however, they are not alleles.
Homozygotes are organisms that contain the same allele on both the homologous chromosomes. Homozygotes can be denoted by AA or aa. AA individuals are called homozygous dominants, since they contain two dominant alleles. While aa individuals are homozygous recessives, because they contain two recessive alleles. True breeding Homozygotes are purebred organisms that are homozygous for certain phenotypes, that always pass down the same trait to off-springs for many generations.

Therefore, from the above discussion it can be concluded that option (A) is correct.

Note: Alleles are the different forms of a gene that occupy the same position on homologous chromosomes. Alleles are present in pairs and influence the same character. Alleles can be dominant or recessive. Dominant alleles obscure the expression of recessive alleles. Hair colour, skin colour, eye colour, different blood groups are all examples of alleles present in the population.